Large cooling installations, such as industrial refrigeration systems or air conditioner systems for office complexes, often involve the use of high cooling capacity systems of greater than 400 refrigeration tons (1400 kW). Delivery of this level of capacity typically requires the use of very large single stage or multi-stage compressor systems. Existing compressor systems are typically driven by induction type motors that may be of the hermetic, semi-hermetic, or open drive type. The drive motor may operate at power levels in excess of 250 kW and rotational speeds in the vicinity of 3600 rpm. Such compressor systems typically include rotating elements supported by lubricated, hydrodynamic or rolling element bearings.
The capacity of a given refrigeration system can vary substantially depending on certain input and output conditions. Accordingly, the heating, ventilation and air conditioning (HVAC) industry has developed standard conditions under which the capacity of a refrigeration system is determined. The standard rating conditions for a water-cooled chiller system include: condenser water inlet at 29.4° C. (85° F.), 0.054 liters per second per kW (3.0 gpm per ton); a water-side condenser fouling factor allowance of 0.044 m2-° C. per kW (0.00025 hr-ft2-° F. per BTU); evaporator water outlet at 6.7° C. (44.0° F.), 0.043 liters per second per kW (2.4 gpm per ton); and a water-side evaporator fouling factor allowance of 0.018 m2-° C. per kW (0.0001 hr-ft2-° F. per BTU). These conditions have been set by the Air-Conditioning and Refrigeration Institute (ARI) and are detailed in ARI Standard 550/590 entitled “2003 Standard for Performance Rating of Water-Chilling Packages Using the Vapor Compression Cycle,” which is hereby incorporated by reference other than any express definitions of terms specifically defined. The tonnage of a refrigeration system determined under these conditions is hereinafter referred to as “standard refrigeration tons.”
In a chiller system, the compressor acts as a vapor pump, compressing the refrigerant from an evaporation pressure to a higher condensation pressure. A variety of compressors have found utilization in performing this process, including rotary, screw, scroll, reciprocating, and centrifugal compressors. Each compressor has advantages for various purposes in different cooling capacity ranges. For large cooling capacities, centrifugal compressors are known to have the highest isentropic efficiency and therefore the highest overall thermal efficiency for the chiller refrigeration cycle. See U.S. Pat. No. 5,924,847 to Scaringe, et al.
In general terms, the compressor comprises an aerodynamic section, a drive train and a control system. The type of aerodynamic section employed depends on several factors including the refrigerant, the required pressure ratio, and the capacity range. The aerodynamic section may have one impeller (single stage) or multiple impellers (multi-stage). A single stage compressor is well suited for comfort cooling applications where the pressure ratio is typically less than 3 and equipment cost is important. Single stage compressors are typically characterized by a consistent cycle efficiency across a broad operating range in comparison to multi-stage compressors.
In a multi-stage compressor, each stage increases pressure of the compressed gas from the exit of the previous stage. Multi-stage compressors can be outfitted with an economizer (aka “intercooler”) to provide a cycle efficiency that is higher than single stage compressors across a narrow operating range, but with added cost and complexity. See “Heating, Ventilating and Air Conditioning Systems and Equipment,” 1996 ASHRAE Handbook (Inch-Pound Edition) and “Fundamentals,” 2005 ASHRAE Handbook (Inch-Pound Edition), both of which are hereby incorporated by reference herein other than any express definitions of terms specifically defined. The greater the pressure ratio required, the greater the efficiency benefit from a multi-stage compressor. Cost is increased due to larger size, the need for a higher quantity of precision components (e.g. multiple impellers, deswirl vanes) and the additional piping and components for the economizer.
The compressor impeller can be either directly driven by the motor or driven through a speed increasing gear set. For high-pressure refrigerants such as HFC-134a, impeller rotational speeds may exceed 3600 revolutions per minute (rpm). Because standard induction motors spin at a maximum of 3600 rpm at a 60 Hz line frequency, a geared speed increaser may be needed to reach rotational speeds exceeding 3600 rpm. The geared speed increaser introduces inefficiencies, including the energy losses that occur in the power transmission through the gears and the viscous losses to the oil.
Alternately, an induction motor can be driven above 3600 RPM synchronous speed by a Variable Frequency Drive (VFD). However, the heat losses attendant the induction principle and resulting inefficiency become excessive when rotating at the high speeds required for refrigerants such as R-134a.
The use of magnetic bearings for enhanced efficiency of a compressor drive system is known. For example, U.S. Pat. No. 5,857,348 to Conry (Conry) discloses the use of active magnetic radial and axial bearings in a centrifugal compressor. Conventional compressor systems utilize hydrodynamic or rolling element bearings wherein the shaft journal is in contact with the rolling elements or a lubricant. Magnetic bearings eliminate such rolling contact or lubricant shear forces and thus characteristically have less drag than lubrication-based bearings.
However, magnetic bearings may be subjected to contact damage whenever the bearing loses power. Such power loss may be routine (and therefore designed around), such as during shut down of the compressor. But some losses of power are unanticipated, such as a power outage or other interruption in power service. In either case, undesired contact can occur, and may result in damage to the bearings or other components that are sensitive to the close alignment tolerances provided by the magnetic bearing levitation.
The conventional wisdom with respect to the design of magnetic bearing, direct drive centrifugal refrigeration compressors is that development of substantially greater capacities is not feasible in a single compressor because of the higher shaft masses and diameters (i.e. higher polar moments of inertia) and power densities.
Another concern in the implementation of magnetic bearings is failure of the bearing controller itself. Often, failure of the bearing controller will result in damage to the rotating components due to uncontrolled movement.
Many high capacity chiller systems feature a motor that operates at a constant operating rotational speed. The operating speed is chosen based on optimum performance at or near full load. However, more advanced control methods often involve frequent operation of the chiller at less than full capacity. The operation of a fixed speed compressor at less than full capacity introduces inefficiencies.
In a centrifugal compressor, refrigerant is motivated through the cooling system by an impeller. Current production impellers often utilize a tapered bore as the means for mounting the impeller to the high-speed shaft. This mounting configuration is inexpensive and has been used successfully for many years. However, there are some inherent problems with the tapered bore mount. For example: (1) alignment of the impeller and shaft is difficult and time consuming; (2) the mounted axial location of the impeller is not repeatable, varying slightly each time the impeller is mounted; and (3) the shaft and impeller combination may require rebalancing after each assembly.
Typically, the motor driving the compressor is actively cooled, especially with high power motors. With chiller systems, the proximity of refrigerant coolant to the motor often makes it the medium of choice for cooling the motor. Many systems feature bypass circuits designed to adequately cool the motor when the compressor is operating at full power and at an attendant pressure drop through the bypass circuit. Other compressors, such as disclosed by Conry, link coolant flow through the bypass circuit to a throttling device that regulates the flow of refrigerant into the compressor. Furthermore, U.S. Patent Application Publication 2005/0284173 to de Larminat discloses the use of vaporized (uncompressed) refrigerant as the cooling medium. However, such bypass circuits suffer from inherent shortcomings.
Some systems cool several components in series, which limits the operational range of the compressor. The cooling load requirement of each component will vary according to compressor cooling capacity, power draw of the compressor, available temperatures, and ambient air temperatures. Thus, the flow of coolant may be matched properly to only one of the components in series, and then only under specific conditions, which can create scenarios where the other components are either over-cooled or under cooled. Even the addition of flow controls cannot mitigate the issues since the cooling flow will be determined by the device needing the most cooling. Other components in the series will be either under-cooled or over cooled. Over cooled components may form condensation if exposed to ambient air. Under-cooled devices may exceed their operational limits resulting in component failure or unit shut down.
Large chiller systems often have specific maintenance requirements related to oiling systems. Where rolling element or hydrodynamic bearings are used, the bearings must be provided with lubrication. Likewise, any gearing that steps up or steps down the speed of the drive shaft must also be provided with lubrication. The oil system provides lubrication to these components, which requires ancillary equipment such as an oil reservoir, a pump, a recirculation loop, an oil heater (to keep the oil viscosity low in the winter months), and an oil cooler (to prevent overheating of the oil in the summer months). These components typically require periodic maintenance such as filter replacement, seal replacement, oil quality sampling, oil replacement and repair of the pump, heater and cooler. The oiling system shares a common atmosphere with the refrigeration components, which typically introduces oil into the refrigerant and can have a detrimental effect on heat transfer. Furthermore, components such as the pump, heater, cooler and recirculation loop may require isolation from ambient atmosphere, which introduces the potential for leak points in the overall refrigeration system.
In order to replace internal compressor components, existing compressor designs often require removal or disassembly of other compressor components that are not scheduled for or otherwise do not require servicing. Reassembly often requires precision alignment procedures that are time consuming and alter the performance of the unit if done incorrectly. In addition, the aerodynamic and motor housings are often contained in a single cast structure, which reduces the ability to change or upgrade aerodynamic components since the size is limited to the existing casting size.
Another characteristic of existing large capacity centrifugal compressors designs is the weight of the assembly. For example, the rotor of a typical induction motor can weigh hundreds of pounds, and may exceed 1000 pounds. Also, as systems are developed that exceed existing horsepower and refrigerant tonnage capacity, the weight of such units may become problematic with regard to shipping, installation and maintenance. When units are mounted above ground level, weight may go beyond problematic to prohibitive because of the expense of providing additional structural support.
There is a long felt need in the HVAC industry to increase the capacity of chiller systems. Evidence of this need is underscored by continually increasing sales of large capacity chillers. In the year 2006, for example, in excess of 2000 chiller systems were sold with compressor capacities greater than 200 standard refrigeration tons. Accordingly, the development of a compressor system that overcomes the foregoing design challenges for delivery of refrigeration capacities substantially greater than the existing or previously commercialized systems would be welcome.